A. Industrial Setting
Numerous industries employ processes which require accurate delivery of a binary gas mixture consisting of a gas of interest and a carrier gas. To achieve accurate delivery, these industries require precise measurements of the concentration of the gas of interest in the flowing gas mixture, where the gas of interest is typically of high purity and may be highly corrosive. Examples of these processes include: chemical vapor deposition, dopant diffusion in, for example, the semiconductor industry, etching, the operation of high efficiency hydrogen cooled generators, and the like.
Current practice in the above and other industries is to use mass flow controllers upstream of a bubbler (vaporizer) to predict (control) the concentration of the binary gas mixture which is generated in the bubbler. This approach suffers from insufficient accuracy due to, among other things: variations in the bubbler temperature; instability of the temperature and pressure of the binary gas mixture; possible leakages in the gas lines upstream and downstream of the bubbler; and concentration time delays between the mass flow controllers and the points of interest, especially at low flow rates.
In addition, the existing equations used to predict a bubbler's pick-up rate are inaccurate. The following is an example of such an equation, where G.sub.A is the mass pickup rate of gas A (the gas of interest), Q.sub.B is a flow rate of gas B (the carrier gas), P is the pressure of the binary gas mixture, P.sub.VA is the vapor pressure of gas A at the bubbler's operating temperature, M is the molecular weight of gas A, T is the temperature of the binary gas mixture, and R is the ideal gas constant: ##EQU1##
This equation can exhibit an inaccuracy as high as 20% when the bubbler's operating temperature is well below the boiling point of gas A, which is the typical operating condition used in practice.
At present there is a wide variety of concentration sensors on the market which use different measurement approaches, including acoustical (EPISON and MINISON devices sold by Thomas Swan of the United Kingdom), optical (IR-5 device sold by MKS Instruments, Inc., Andover, Mass.), thermal conductivity (Varian Model 3400 Gas Chromatograph, sold by Varian Vacuum Products, Lexington, Mass.), and mass spectroscopy. None of these approaches fulfill all the requirements for a binary gas measuring system, including robustness, maintenance free operation, and the ability to produce highly accurate and repeatable real time concentration measurements of high purity and/or highly corrosive gaseous media.